The performance of W-CDMA communications systems employing fast power control is generally dependent on an inner-loop power control algorithm, which requires an estimation of a signal-to-interference ratio (SIR), also referred to herein as the signal-to-noise ratio (SNR), at the output of the rake receiver. In W-CDMA communications systems, the mobile user equipment (UE) transmits to the network a power control command based on estimated SNR, at a 1500 Hz (slot) rate, for use in controlling transmission power at the network. The estimated SNR is also used by mobile user equipment for base station selection.
It is known to estimate the signal to noise ratio based on the ratio of a biased signal power estimator and a biased noise power estimator. The problem associated with this known SNR estimator is that it is characterized by a strong bias and substantial variance. Merely providing unbiased signal and noise estimators is generally insufficient to ensure that the resulting SNR estimator will be unbiased. Furthermore, since bias is a function of the signal to noise ratio, the performance of the SNR is potentially sensitive to the actual signal and noise power, particularly under diverse channel conditions, for example, those typical of multi-path fading propagation channels in wireless communications systems.
WO 99/66643 entitled “Device And Method For Measuring Non-Orthogonal Noise Power For CDMA Communication System” discloses detecting non-orthogonal noise power by squaring a difference between adjacent symbols.
Beaulieu et al. proposes in a publication entitled “Comparison of Four SNR Estimators for QPSK Modulations” IEEE Communications Letters, Vol. 4, No. 2, February 2000 an empirical method for compensating for over-estimation of SNR that results in an approximately unbiased SNR estimator when the SNR is large and the channel, which includes additive white Gaussian noise (AWGN), is not fading. However, the SNR level on each path of multi-path fading propagation channels typical of wireless communications systems are usually relatively low. Therefore, the scheme proposed by Bealieu is unsuitable for unbiasing an estimated SNR in wireless communications applications.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.